With the thumb out it means "I love you"
With the thumb IN, it's the devils horns...In Italy (back in the day), when someone wanted someone cursed, they would point the horns at them...
Ronnie James Dio who is an old Metal artist used to see his grandma doin this when she was scolding him and he thought it looked cool so he started throwin it up at his concerts....Pretty soon it was a metal icon!